(1) Field of the Invention
The present invention relates to an optical modulator and an optical modulating method suitable for use when an optical signal is modulated in, for example, a terminal apparatus in an optical communication system.
(2) Description of Related Art
In recent years, it is required to transmit an enormous volume of information, with development of a highly information-oriented society. As means for transmitting such an enormous volume of information, there are used optical communication systems transmitting information as optical signals.
In the optical communication system, a higher transmission speed is required year after year since a modulation rate of signals is increased more and more. For this, optical devices of an optical waveguide type such as external modulators or the like for high-speed modulation of signals are used in various places.
In the optical modulator used in such an optical communication system, there are expectation for a higher modulation rate and a demand for a lower voltage used to drive the optical modulator in order to decrease a size of a chip of the optical modulator. Namely, there is a demand for an optical modulator which can modulate light at a high-rate and can be driven at an extremely low voltage.
In order to drive a known optical modulator at a low voltage, there are assumed two manners. Namely, the modulator is driven at a low voltage [V.sub.0 -V.sub.1 shown in FIG. 16(a)] in the vicinity of 0 output of an output light power as shown in FIG. 16(a), or the modulator is driven at a low voltage [V.sub.0 '-V.sub.1 ' shown in FIG. 16(b)] in a portion where a slope of the output light power is the steepest, as shown in FIG. 16(b).
However, when the known optical modulator is driven at a low voltage in the vicinity of 0 output of the output optical power, it is impossible to obtain a large modulated optical signal since the output optical power is a sine wave waveform, as shown in FIG. 16(a). When such the modulated optical signal is used in the optical communication system, information in the optical signal is lost because of degradation of the optical signal waveform upon transmission.
On the other hand, when the optical modulator is driven at a low voltage in a portion where the slope of the output optical power is the steepest, an extinction ratio is degraded since a direct current light [indicated by reference character d in FIG. 16(b)] is superimposed on the modulated optical signal although a large modulated optical signal can be obtained, as shown in FIG. 16(b).